How To Do
The Rule of 72: Estimate How Fast Your Money Doubles
There’s a mental math shortcut that every investor should know. It fits in one sentence: divide 72 by the annual return rate, and you get the approximate number of years for your money to double.
The Formula
Years to double = 72 / Annual return rate
That’s it.
Quick Examples
| Return Rate | Years to Double |
|---|---|
| 6% (FD) | 12 years |
| 8% (Debt fund) | 9 years |
| 10% (Balanced fund) | 7.2 years |
| 12% (Equity fund) | 6 years |
| 15% (Aggressive equity) | 4.8 years |
| 18% (Small cap, if lucky) | 4 years |
At 12% returns, your money doubles every 6 years. That means 1 lakh becomes 2 lakhs in 6 years, 4 lakhs in 12, 8 lakhs in 18, and 16 lakhs in 24. One lakh turned into sixteen without adding a single rupee.
How Accurate Is It?
Very. The rule gives exact results at ~8% and stays within a few months of the actual doubling time for rates between 4% and 20%.
| Rate | Rule of 72 says | Actual years |
|---|---|---|
| 6% | 12.0 | 11.9 |
| 10% | 7.2 | 7.3 |
| 15% | 4.8 | 5.0 |
| 20% | 3.6 | 3.8 |
Close enough for any practical purpose.
Using It in Reverse
The rule works both ways. If your investment doubled in 5 years, your CAGR was approximately 72 / 5 = 14.4%.
This is great for quickly evaluating claims. Someone says their portfolio went from 10L to 40L in 10 years? That’s two doublings (10 to 20 to 40), so 5 years per doubling, which means about 14.4% CAGR. Solid but not extraordinary.
The Dark Side: Inflation
The Rule of 72 works for inflation too. At 6% inflation, your money’s purchasing power halves every 12 years. That FD earning 6% isn’t growing your wealth. It’s running in place.
For real wealth growth, your investments need to beat inflation by a meaningful margin. A 12% return with 6% inflation gives you a real return of roughly 6%, doubling your purchasing power every 12 years.
Why This Matters
The Rule of 72 makes compound interest tangible. Instead of abstract percentages, you can instantly see how different rates stack up over your investing lifetime. That 2% difference between an 8% and 10% return doesn’t sound like much, until you realize it’s the difference between your money doubling every 9 years versus every 7.2 years. Over 30 years, that gap is enormous.